Leptin, a protein occurring naturally in the blood, appears to

Step 1: Identify the Question

The argument contains a blank space, and the question asks which answer choice most logically completes the passage. Fill-in-the-blank questions are typically Strengthen questions; however, to be sure of what problem type this is, read it and determine the logical relationship between the blank and the rest of the passage.

Step 2: Deconstruct the Argument

L: regulates fat (met. ↑, app. ↓)

Mice w/ no L: fat ↑, BUT L injections → fat ↓

BUT: L not a dietary supp. for fat, b/c ??????

The first part of the argument describes the fat-regulating properties of leptin. It appears to reduce fat when injected into mice. But, there’s a catch: surprisingly, it can’t be used as a dietary supplement. The argument doesn’t explain why this is; instead, that section is left blank. Because you’re asked to explain a surprising fact with no prior evidence, treat this as an Explain the Discrepancy problem.

Step 3: Pause and State the Goal

On Explain the Discrepancy problems, the right answer will give a reason for the surprising phenomenon to have occurred. In this case, the right answer needs to give a valid reason that leptin can’t be used as a dietary supplement.

Step 4: Work from Wrong to Right

(A) CORRECT. This explains why leptin can’t be used as a dietary supplement: if it’s eaten, it is broken down by the digestive system and can’t enter the bloodstream. Therefore, it would be ineffective. This is consistent with the statements in the argument: leptin naturally occurs in the blood, and is effective when injected.

(B) The surprise in this argument is specifically that leptin is ineffective. Whether other pharmaceuticals have similar effects doesn’t bear on whether leptin itself is effective.

(C) If anything, this suggests that leptin is a useful supplement, which is the opposite of what the right answer should do. However, it also doesn’t directly relate to the argument, since it doesn’t clarify whether leptin is an effective supplement—it only describes naturally occurring leptin.

(D) The mice in the experiment were injected with leptin, not given leptin as a dietary supplement. Therefore, information about the mice doesn’t help determine whether leptin is effective as a dietary supplement.

(E) The mice in the experiment were injected with leptin, not given leptin as a dietary supplement. Therefore, information about the mice doesn’t help determine whether leptin is effective as a dietary supplement. Additionally, this suggests, if anything, that leptin is effective, which is the opposite of what the right answer should do.